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Σ or Sigma is the Greek upper case capital letter S and is used in mathematics to represent summation or addition of a series of elements or set of data values.
x₁, x₂, x₃, ... x_n is a set of numbers. x₁ is the first number in the set, x_i is the 'i'th number, x_n is the last of n numbers.
Elements can be simply adding up a set, or more involved such as summing squares or other algebraic equations.

Haruhiko 'Harry' Matsumura wrote a comment "Hi, Sirs, Regarding square root law of inventory, I prepared a useful kaizen proposal paper to give my clients to address stock reduction by means of location."

Machine cycle time (T_cm) represents the actual time it takes for one machine to complete all of its operations on one piece, product, patient, file, etc.

Every Part Every Interval, also known as EPEI or EPEx, represents the frequency that different parts are produced or services provided within a fixed repeating schedule. This fixed repeating schedule is often graphically portrayed, for training purposes and as a scheduling visual control, as a wheel, with the different products represented by alphas (A, B, C…) and the wheel indexed clockwise to follow the intended sequence.

When most people hear the word average they think normal, typical, boring. But, in many cases that is very far from the truth. 

Averages can not only be interesting, but also impossible!

Consider the call center that measures average response time. The average response time is less than their target, so they must be doing pretty good, right??? 

Wrong! 

I am saddened to learn of the passing of George E.P. Box. I never met him, but like many, I knew him through his work.  I have read and reread several of his books. They are some of my most prized possessions.

The seminal book on lean, The Machine that Changed the World, spent many words, tables, and figures on the subject of productivity (as well as, of course, quality).

Why?

Productivity is one of the critical few measures that reflect the “leanness” of a process, value stream or enterprise. It captures how effectively an organization uses its resources, and it’s usually a meaningful way to compare performance over time and between entities.

Lean oriented questions tend to be straightforward, but not necessarily easy.